The present invention is directed to a set screw for tangentially engaging a spine fixation rod and, more particularly, to such a set screw having a pair of coaxial or co-linear screw shafts with different diameters and separated by an abutment or bearing surface to urge the rod into frictional engagement with the inner surface of a receiving member and to positively engage the surface of the rod to prevent axial and non-axial displacement of the rod and rotation of the rod relative to the receiving member.
The art of correcting back deformities, injuries and the like has advanced dramatically in recent years. Surgeons who perform operations on the spine and related bones of the body are constantly developing new procedures and techniques that require implants which are capable of being stable in the body after implantation and which firmly lock in place to resist the substantial pressures and forces developed by the body on the implant. Such implants must often also resist twisting or torsion applied to parts of the implant, and certain prior art implants have not always been successful at resisting twisting or torsion.
Such implants often involve rods which are placed along the spinal column or various bones of the living body and preshaped or, once secured to the bones, such rods may be bent and/or further shaped to force the bones to align with the rods and, in this manner, provide correction to misalignment of the bones or bone parts caused by deformity, injury or the like. That is, one or more rods are placed in the correct curvature for the spine and the spinal bones are joined to the rod or rods, before or after bending the rods, in such a manner as to thereafter force the bones to follow the same configuration as the rods.
Once the rods are bent, forces created by the muscles of the patient, by sudden movement, by accident, or the like often act to try to rotate or apply torque to the implant as a whole or to a rod individually. Such forces generally apply torsion to the apparatus which may act to loosen or even dislodge the implant or to turn or rotate one or more rods to a less effective support position. It is desirable that the apparatus be able to resist such forces acting upon it.
Historically, the rods used as implants in the manner described above, are typically joined with various bones along the length of the rod by use of bone screws or other implants that are joined with the rod. It has been found that conventionally available implants join rods to bone screws or intermediate connectors in such a manner that the rods are often held against axial movement relative to the bone screws or intermediate connector. That is, the rods are not likely to move substantially with respect to the other implants in a direction that is along the central or longitudinal axis of the rod. However, because of the substantial forces exerted on the rod during use, certain forces act to try to rotate the rod within bone screws and connectors, such that the spinal corrective configuration and positioning of the rod can slip due to rotation of the rod from an optimal position to one that is less suited for the patient. This can occur when substantial forces are applied to the back during exercise, accident or the like.
Consequently, it is desirable to have an implant that not only effectively resists axial movement of the rod relative to the other implants, but also effectively resists torque or torsion that produce turning of the rod or rotation of the rod relative to the implants. One use of the present invention is especially suited for the locking and stabilizing a posterior spinal implant system having at least one, but normally a pair of elongate rods that extend along the spine. In particular, the installation of spinal rods is often utilized to reposition the spine and correct deformities and the like. Such a rod is typically anchored at opposite ends to vertebrae and is likewise joined with vertebrae along the length of the rod by bone screws or the like. The system is typically installed by curving the rod to fit the malformed spine of the patient and then securing the anchors at both ends and various intermediate bone screws to the rod. The rod is thereafter bent by rod bending tools to assume the desired configuration of the spine and the rod in this manner translates the various bones of the spine along with it to the correct configuration.
Once the rod is bent, the body exerts forces, including substantial rotational forces or torsion on the rod, especially should the patient be struck on the back, during exercising, or the like. It is also noted that the rod can first be bent and then the bone moved to the rod and secured to the rod. In either case, it is important that the anchors at opposite ends of the rod and at the bone screws along the rod resist rotation of the rod and that the anchors themselves remain stable and securely attached to an associated bone. Furthermore, it is important to both lock the rod against rotation in or relative to the bone screw and to secure the anchors of the rod against rotation relative to the spine.
An implant system is therefore desirable that provides a strong anchor at opposite ends of the rod and at bone screws along the rod that resists rotation of the rod both relative to the bone screws and relative to the spine during procedures at the time of implantation and later during use and that such a system also resist axial displacement of the rod relative to anchoring bone screws and other bracing members used in the system.
The present invention provides a multiple diameter tangential set screw with an intervening concave, radiused abutment surface for securing a rod against axial and rotational movement relative to a member in which the rod is received. The present invention has particularly advantageous application in implanted bone and spinal fixation components. The rod receiving member may be an open hook type connector of a rod anchor, a cross connector, or other type of element to which a rod is to be secured in the manner wherein a rod is biased into a hook like receiver and then held therein against radial, axial or rotational relative movement. Alternatively, the rod receiver can be a closed type of element. The rod receiver has a rod receiving opening with a surface that is in at least partially surrounding relation to the rod when the rod is in the receiver. A threaded set screw bore is formed in partially intersecting relation to the rod receiving opening.
The set screw has a threaded mounting segment which has a diameter and threading compatible with the set screw bore. A threaded rod engaging segment extends coaxially from the mounting segment and has a reduced diameter relative to the mounting segment. An abutment or bearing surface provides a transition between the mounting segment and the rod engaging segment. In a preferred embodiment of the present invention, the abutment surface is a concavely radiused surface of revolution with a radius substantially equal to the radius of the rod with which the set screw will be used.
The position and relative angle of the set screw bore and the radius of the rod engaging segment are configured in such a manner that the abutment surface forces the rod into frictional engagement with the surrounding surface of the rod opening to thereby clamp the rod between the rod receiver and the abutment surface of the set screw. Such clamping resists relative radial movement and axial movement of the rod along its longitudinal axis and, to some extent, resists rotational movement of the rod within the rod receiver. The threads of the rod engaging segment of the set screw cut into the surface of the rod to positively resist and secure the rod against rotational movement.
In an implant implementation of the present invention, the set screw is formed with a break off driving or installation head, such as of a hexagonal configuration, which is connected to the mounting segment of the set screw by a weakened cross section or torque limiting region. The weakened area causes the installation head to break off the mounting segment at a preselected torque. The hex head facilitates use of a compatible installation tool to rotate and thereby advance the set screw into clamping engagement with the rod. The mounting segment of the set screw may also be provided with formations for engagement by a removal tool, such as a hexagonal socket and/or a set of peripheral slots that are parallel to the axis of rotation or the like.
Therefore, the objects of the present invention include: providing improved components for implanted bone fixation systems and, particularly, spinal fixation systems; providing improved fasteners for securing fixation rods to various other fixation components; providing, particularly, an improved set screw configuration for securing a connection joint between a rod and a rod receiving member; providing such a set screw which engages the rod and the rod receiving member frictionally and in such an interfering manner as to positively secure the rod within the receiving member; providing such a set screw which secures the rod against both axial and rotational movement relative to the rod receiver; providing, particularly, a multiple diameter set screw which is positioned at a partially intersecting or substantially tangential relation to the rod; providing such a set screw including a threaded mounting segment sized and threaded to fit within a threaded bore formed in the rod receiver member; providing such a set screw including a threaded rod engaging segment extending coaxially from the mounting segment and having such a diameter sized and shaped to interferingly or threadedly engage the surface of the rod, that is, to cut into the surface of the rod to positively fix the position of the rod within the rod receiver; providing such a set screw including an abutment or bearing surface which transitions between the mounting segment and the rod engaging segment and that is configured to engage the rod and urge it into frictional engagement with the surrounding surface of the rod receiving member; providing such a set screw in which the abutment surface is a concave radiused surface of revolution having a curvature or concave radius substantially equal to that of the rod to be secured within the rod receiver; providing such a set screw including a weakened section to form a break-off installation head which is adapted to separate at a preselected torque applied by an installation tool to the screw head; providing such a set screw including removal structure or formations to enable positive engagement by a removal tool for removal of the set screw from the rod receiver; providing such a set screw configuration which has advantageous application in implanted spinal fixation systems; providing such a set screw which is adaptable to applications other than such implanted systems; and providing such a multiple diameter tangential set screw which is economical to manufacture, which is effective in use, and which is particularly well adapted for its intended purpose.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.